A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Single-Atom Catalysts across the Periodic Table.

Supported metal oxide and other catalysts for ethane conversion: a review

http://iglesia.cchem.berkeley.edu/Publications/JournalofCatalysis_206_202_2002.pdf

Promoted Iron-Based Catalysts for the Fischer–Tropsch Synthesis: Design, Synthesis, Site Densities, and Catalytic Properties

Incorporation of gallium into zeolites: syntheses, properties and catalytic application.

The Chemical State of Gallium in Working Alkane Dehydrocyclodimerization Catalysts. In situ Gallium K-Edge X-Ray Absorption Spectroscopy

A determination is described of copper metal, oxide, and hydroxide in samples containing both nickel and copper. The analysis was based on K-edge x-ray absorption spectra of the copper. This allowed us to consider the chemical states of copper and ignore the nickel also present. The method involved fitting linear combinations of copper standard spectra to the spectrum from copper in the sample. The fit composition that minimized the variance gave the sample composition. The work demonstrated that K-edge x-ray absorption spectra are sufficiently characteristic to allow their use for quantification of components in mixtures. This method of analysis is complemented by diffraction and less general techniques. It is a strength of x-ray absorption edge analysis that it is element-specific and applicable to most elements maintained under any conditions of temperatures, pressure, and atmosphere.

Analysis of mixtures of compounds of copper using K-edge X-ray absorption spectroscopy

Hydrogen removal occurs by recombinative desorption and by hydrogen transfer during dehydrogenation steps required in the conversion of alkanes on H-ZSM5 and on H-ZSM5 modified by Ga or Zn. On H-ZSM5, hydrogen removal limits the rate of propane conversion to aromatics. The resulting high surface hydrogen fugacities lead to cracking, a step that removes both adsorbed hydrogen and deactivating residues. Ga and Zn ions catalyze the recombinative desorption of H-adatoms, lower hydrogen surface fugacities, and allow acid sites to turnover without cracking. Metal ions act as “portholes” for the desorption of H-atoms as H 2 . In-situ EXAFS neasurements show that Ga +3 ions reduce during reactions of propane or pretreatment by H 2 . Hydrogen desorption and propane aromatization rates depend on the density of reduced Ga species in H-ZSM5, which catalyze rate-limiting hydrogen removal steps. The overall catalytic sequence is bifunctional; Bronsted acid chemistry is apparently enhanced by the presence of sites that stabilize hydride ions and catalyze H + -H − recombination reactions.

The Role of Surface Fugacities and of Hydrogen Desorption Sites in Catalytic Reactions of Alkanes

Ga species in H-ZSM5 zeolites catalyze rate-limiting dehydrogenation steps during the conversion of alkanes to aromatics. Specifically, they promote the recombinative desorption of H-adatoms as H2 and thus inhibit undesired cracking reactions. Here, we describe studies of the physical and chemical state of Ga using in-situ X-ray absorption at the Ga K-edge. Ga+3 species initially present in fresh catalysts reduce at temperatures below 770 K during hydrogen pretreatment or propane reactions. Reduced Ga is present in highly dispersed form without Ga nearest neighbors, probably as a monomeric hydride species coordinated to basic oxygens within zeolite channels. These hydride species are driven to release molecular hydrogen by the high surface hydrogen fugacities that develop during propane dehydrocyclodimerization on H-ZSM5. Reduced Ga species reoxidize to Ga+3 when samples are cooled to room temperature in flowing H2, suggesting that active forms of Ga exist only at reaction conditions. This work illustrates the critical need for in-situ techniques to establish the chemical form of catalytic sites and the misleading conclusions that can arise from the exclusive use of pre- and postreaction characterization to suggest the nature of the catatlytic sites.

G.D. Meitzner

Papers

The Chemical State of Gallium in Working Alkane Dehydrocyclodimerization Catalysts. In situ Gallium K-Edge X-Ray Absorption Spectroscopy

Analysis of mixtures of compounds of copper using K-edge X-ray absorption spectroscopy

The Role of Surface Fugacities and of Hydrogen Desorption Sites in Catalytic Reactions of Alkanes

The Chemical State of Gallium in Working Alkane Dehydrocyclodimerization Catalysts. In situ Gallium K-Edge X-Ray Absorption Spectroscopy.